Noise reduction is the process of removing noise from a signal. Noise may be any undesirable sound that is present in the signal. Noise reduction techniques are conceptually very similar regardless of the signal being processed, however a priori knowledge of the characteristics of an expected signal can mean the implementations of these techniques vary greatly depending on the type of signal.
All recording devices, both analogue and digital, have traits which make them susceptible to noise. Noise can be random or white noise with no coherence, or coherent noise introduced by a mechanism of the device or processing algorithms.
In electronic recording devices, a form of noise is hiss caused by random electrons that, heavily influenced by heat, stray from their designated path. These stray electrons may influence the voltage of the output signal and thus create detectable noise.
Algorithms for the reduction of background noise are used in many speech communication systems. Mobile phones and hearing aids have integrated single- or multi-channel algorithms to enhance the speech quality in adverse environments. Among such algorithms, one method is the spectral subtraction technique which generally requires an estimate of the power spectral density (PSD) of the unwanted background noise. Different single-channel noise PSD estimators have been proposed. Multi-channel noise PSD estimators for systems with two or more microphones have not been studied very intensively.